1. Field of the Invention
This invention relates to fuel injectors for gas turbine engines, and particularly to a new and improved low profile fuel injection system for injecting fuel into a thrust augmenter of a gas turbine engine.
2. Description of the Prior Art
A thrust augmenter, or afterburner, is added to a gas turbine engine in order to increase the engine thrust. An augmenter typically comprises a casing added to the aft end of the engine with an arrangement of flame holders, fuel injectors, and appropriate fuel manifolds. When augmented thrust is desired, fuel is injected through injector tubes into the gas exiting the turbine section of the engine. The fuel-gas mixture is ignited and the resulting combustion produces additional thrust.
An augmenter can also be utilized on the aft end of a turbofan engine. In such an arrangement, fuel can be injected not only into the gas exiting the turbine, but also into the bypass air exiting the fan duct.
One difficulty which has been encountered with augmenters, however, is their radial size. A high performance aircraft requires a minimum frontal area in order to decrease drag and increase speed and range. Correspondingly, this requires an engine nacelle of minimum frontal area as well. Prior art augmenters, however, include components which extend radially outward to such a distance that the engine nacelle must be enlarged radially to accommodate them. Thus, the desired minimum engine nacelle size is unobtainable. Furthermore, if it is desired to modify an engine already fitted to an aircraft by adding an augmenter to the engine, the modification may be made impractical because of the necessity of increasing the dimensions of the nacelle to accommodate the augmenter.
More specifically, one component of the augmenter having a radial extension contributing to the size problems is the fuel injection system. A typical fuel injection system of an augmenter includes injector tubes, injector valve heads including fuel-metering means and from which the injector tubes extend, fuel manifolds carrying the fuel to the injectors, and connecting conduits which transfer the fuel from the manifolds to the injector valve heads. The injector valve heads, fuel manifolds, and connecting conduits of existing fuel injection systems are mounted on the outer surface of the augmenter casing in such a manner that the portion of the nacelle in which they are located must be significantly enlarged radially relative to other portions of the nacelle.
Another difficulty encountered in prior augmenters is that the augmenter casing often requires ring stiffeners or other stiffening provisions to maintain casing strength and rigidity. Such stiffeners not only add to engine cost and weight, but can also increase the radial dimensions of the augmenter.
Still another problem with prior augmenters involves assembly and maintenance. Most augmenters include multiple injector tubes, valve heads, and connecting conduits spaced around the circumference of the casing. Connections between each component are typically made with bolted flanges or threaded tube fittings. Installation or replacement of a single injector valve head or an injector tube can thus require multiple suboperations involving other components as well. This increases maintenance time and costs.
Another problem arises when fuel conduits in the form of tubes are mounted transversely on the inner surface of the augmenter casing in a stream of air. This can occur, for example, when a cooling duct carrying a portion of the fan bypass air is installed adjacent the inner surface of the augmenter casing to help prevent casing overheating. Although such mounting helps to maintain a minimum radial dimension of the augmenter, the tubes so mounted increase aerodynamic drag.
Another problem encountered with augmenters on turbofan engines is the excessive number of fuel metering valves required to meter fuel to both the fan duct injector tubes and the core duct injector tubes, particularly when there are twice as many fan as core injector tubes. A suitable manner of reducing the number of metering valves required for an augmenter to a turbofan engine is disclosed in U.S. Pat. No. 3,793,838 assigned to the same assignee as the present invention. The present invention utilizes the same concept of a fuel metering valve in each fan injector valve head being used to meter fuel for fan injector tubes extending from both that fan injector valve head and from adjacent core injector valve heads. This is accomplished by means of jumper tubes. The present invention, however, improves that concept in a low profile configuration.
In view of the above-mentioned problems with prior art augmenters, it is, therefore, a primary object of the present invention to provide a new and improved low profile fuel injection system for an augmenter to a gas turbine engine which permits an engine nacelle containing the augmenter to be of minimum frontal area.
Another object of the present invention is to provide a fuel injection system for an augmenter in which the components of the system provide improved structural strength to the augmenter casing and eliminate the need for casing ring stiffeners.
Another object of the present invention is to provide a fuel injection system for an augmenter in which installation or replacement of injector valve heads or injector tubes is accomplished in a simple operation.
Another object of the present invention is to mount the fuel conduits of a fuel injection system in such a manner as to minimize aerodynamic drag.
Still another object of the present invention is to provide a low profile fuel injection system for an augmenter in which fuel metering for the injector tubes is accomplished with a reduced number of fuel metering valves.